Accurate knowledge of white and green liquor composition is necessary for close control of kraft pulping and recausticizing operations. First, if changes in the green liquor composition can be monitored, feed forward control of the lime feed rate in the causticizing plant can be achieved. Second, compositional information of the white liquor can be used as feedback to compensate for variations in the lime quality or reactivity, and as feed forward compensation for pulping.
In the past, white and green liquor compositions have been determined by laboratory titrations. However, both the analysis rate and the accuracy of routine titrations are not sufficient to take full advantage of modern control systems and strategies.
The Wallin U.S. Pat, No. 3,941,649 describes an attempt to control the pulping time and pulping temperature by taking a sample of the pulping liquor after initial digestion has occurred. The pulping sample is titrated to provide an alkaline content of the liquor. From this alkaline content, the pulping intensity expressed as "H" factor is determined and used to obtain the desired KAPPA number.
The Hultman et al U.S. Pat. No. 4,236,960 describes a process for controlling the degree of causticization of white liquor. The process of the Hultman et al patent includes determining the sodium carbonate concentration of green liquor fed to the causticization, then determining the sodium carbonate concentration of white liquor resulting from the causticization and thereby controlling the degree of causticization within a predetermined range while taking both sodium carbonate concentrations into account.
The Bertelsen U.S. Pat. No. 4,536,253 describes a process for controlling the properties of white liquor by measuring the electric conductivity of the green liquor before causticization in addition to measuring of the conductivity of the white liquor. The conductivity of the green liquor is measured both before the slaker and gradually as it passes through the slaker to determine the reaction of the carbonate.